C2: Simplify control inputs for BarrierSetC2::obj_allocate

Simplify control inputs for BarrierSetC2::obj_allocate. In most cases, it doesn't change anything since `toobig_false` is equivalent to `ctrl`. In rare case, `toobig_false` is created for Unsafe.allocateInstance while instance size is not statically known, `ctrl` would become control input of IfNode whose projects are `toobig_false` and `toobig_true`, old eden_end and old_eden_top can simply accept `toobig_false` as their control input rather than `ctrl`.